Combined video gain and frequency response control



July 24, 1956 E. l. ANDERSON COMBINED VIDEO GAIN AND FREQUENCY RESPONSE CONTROL Original Filed June 22, 1948 UnitedStates Patent O COlVIBINED VIDEO GAIN AND FREQUENCY RESPONSE CONTROL Earl I. Anderson, Port Washington, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Original application June 22, 1948, Serial No. 34,373, now Patent No. 2,627,022, dated January 27, '1953. Divided and this application November 8, 1951, Serial No. 255,410

4 Claims. (Cl. 17 9-171) This invention relates to wave translating circuits applicable to radio receiving equipment, and more particularly to renements in circuits for detecting and amplifying television signals. This application is a division of the copending application Ser. No. 34,373, filed July 22, 1948 and bearing the same title, and now Patent No. 2,627,022 dated January 27, 1953.

The conventional type of present day radio receiver employed for the reception of commercial communication signals, in addition to the incorporation of a mechanism for manually controlling the overall gain of the radio receiver, usually includes some form of automatic gain control as well as some means for manually controlling the frequency response of that particular receiver circuit handling the demodulated signal.

The advantages to be derived from proper automatic gain control action in radio wave receivers used for the reception of sound modulated signals, of course, have their counterpart in equipment designed to produce television images through the reception and detection of radio frequency television signals. In fact, fading or any other irregular signal strength variation of television signals commonly produce visible evidence in the reproduced image, which to the user may constitute considerable more annoyance than mere changes in audio level which result from variations in signal strength applied to sound broadcast receivers. Hence, a number of methods have been devised for applying automatic gain control (hereinafter referred to as A. G. C.) to television receivers and in their practice there have been established definite advantages in using a threshold type of control action.

In superheterodyne circuits employing second detectors of the simple diode variety, one of the more outstanding benefits to be derived from the threshold A. G. C. is the realization of full receiver sensitivity during reception of very Weak signals. Since the linearity of the diode detector is very poor for low operating voltages, it is considered desirable that a signal voltage in the order of two to four volts be available at the output of the last intermediate frequency amplifier for satisfactory reception of any signal. With a threshold or delayed A. G. C. arrangement, no A. G. C. voltage is developed for reducing the sensitivity of the receiver until a signal in excess of a predetermined threshold value, necessary for the development of the required two to four volts by the stage driving the detector, is applied to the receiver terminals.

In television receivers, designed for the reception of negative transmission television signals, it is evident that any serious overload of the receiver circuit, due to excessive signal strengths, will tend to distort the video signal synchronizing information to produce consequent deleterious eects on sweep circuit synchronization. Accordingly,

threshold A. G. C. offers another desirable feature in that it can be designed such that overloading of the last intermediate frequency amplilier is less likely to occur. To achieve this overload protection however, unless con- "ice siderable A. G. C. gain is provided at a consequent additional cost of circuit construction, it is usually desirable to provide means for decreasing the A. G. C. threshold level when operating the receiver under higher signal strengths. Such a threshold decrease is most expeditiously accomplished on a manual basis.

In television receiver circuits having a pass band substantially less than the 4 mc., which the present R. M. A. system allows, experience has shown that a much snappier or crisper picture results if the overall response of the video circuit is not at but made to n'se with frequency. Furthermore, this rising frequency response may provide an apparent improvement in images repre- Senting the at reproduction of a full 4 mc. band width when such an image is viewed at a distance of several feet away. This latter effect may be attributed to the falling olf at high frequency of a good number of presently used camera tubes. However, the increase high frequency video response does tend to produce transients which appear to sharpen all video detail in reproduced television images. For this reason, high frequency boost circuits have been included in the video amplifier portion of numerous television receivers. It must be noted, however, that the tilting of the video amplifier response to accentuate the higher frequencies, increases the apparent amount of noise in the picture since signal noise components are generally of a high frequency order. Thus the magnitude of high frequency boost is preferably made an increasing function of the received signal so that it is more effective when signals are strong and above the internally generated receiver noise, and reduced when receiving weaker signals.

In an embodiment of the present invention, there is provided a manually adjustable degenerative gain control circuit for use in the video amplier section of a television receiver. By novel circuit arrangements this gain control circuit is adapted to additionally provide a control over the video amplifier high frequency `accentuation, such that for high signal levels, the gain of the receiver may be manually reduced with simultaneous automatic increase in the video amplifier high `frequency response.

It is an object of this invention to provide a method of television receiver operation whereby a receiver may be made to exhibit improved response to received variations in signal strength and the economical incorporation of apparatus acting in accordance with this invention is permitted in present day television receivers.

It is another object of the present invention to provide a degenerative manual gain control for television receivers which is of such form as to lend itself readily to the simultaneous control of the degree of frequency response alteration imparted to the receiver video amplifier for a given degree of established receiver gain, such that improved image appearance is realized for strong signal strength signals.

Another object of this invention resides in the provision of a form of manually operated video frequency amplifier gain and frequency response control in combination with A. G. C. threshold control, such that operation of the receiver at higher signal levels is enhanced by a reduction of video frequency amplifier gain which automatically effects an increase in the video amplifier high frequency response and an increase in the A. G. C. threshold level.

It is still another object of this invention to provide a form of television receiver video amplier high frequency accentuation which is automatically varied and controlled by the television receiver manual gain control which is incorporated therein to supply the optimumamount of high frequency boost for any signal intensity. Other objects and advantages of the present invention will become apparent, to those skilled in the art, from y perusal of the following. specification. taken. in connection with the related drawings, the gures of which are merely to be illustrative of preferred forms of the invention and not; limited thereto:

Figure 1 is a circuit and block representation of a television receiver employing the present invention;

Figure 2 is a graphical representation of. the operating characteristicsof that portion of a televisionl receiver embodying the present invention; and

Figure: 3 is a schematic representation of another form of the present invention.

Referring now to Figure l, there is showna television receiverhaving anV antenna 10, a radio frequency ampli fier: a. first detector 14 for converting theV received radio: frequencyJ signal t'or a fixed intermediate frequency, and an interme'diate'frequency amplifier' 156' including one or more.` stages'` of vacuum tube amplification. Circuit details. of: the: intermediate frequency amplifier 16A are shown in part by the cutaway section ofV block 16 which discloses only theflast stage of amplification employing vacuum. tube: 18' with its plate load 201 connected with a source: of positive plate supply potential' 22.

R. F. amplifierv 12, converter 14, and I. F. amplifier 16. may; for example, take the form employed in the presently' marketed television receivers shown and described-in an articleA entitledr Television Receivers publishedin'. the RCA-Review for March 19'47 on page 5.

v The. output of tube 18 is4 coupledtlirough capacitor24 to the anode: 26 of the second detector diodeI 2S. The

signals 30.' appearing'on the anode` 26 is demodulated by the diode. 28 with its associated load impedance 32, 38, and 40 to supply.- the videomodulation envelope shown at.V 42 to the. grid 44- of video amplifier 4'6. A D. C. return peaking inductance 48 for the diode28'is connected to the anode 26 thereof while bias potential for the control,- grid 44' ofvacuum tube 46y is applied through the diode load elements 3`8 andy 40 from the bias voltage sourcei 50:Y The anode or output' circuit of the video amplifier 46 conventionally includes inductances 52 and 54 whichk are suitably chosen to impart to the amplifier 46v substantially flat response to the highest video frequen'cy reproduced, which is normally in the range ofv 3 to 4' mc;V The operating place potential for4 the vacuum tube. 46'- is supplied from a positive source having its' terminal shown at 56. The video signal for the actuation of the knescope 58 is derived from the compensating peaking' network 52'and 54 through the capacitor'll, and is appliecb to the control grid 62V of: the kine'scope throughtthe-Dl Ci restorercircuit 64. Also connected withtlie` control? gridv 62V ofthe kinescopev 58 is a variable bias source 66 employed as' an image brightness control. Now." examining in more detail the circuit configurationy associated with the video amplifier vacuum tub'e- 46, with Whiehl thepresent invention is more directly' concemedgit is-.seen that the'v cathode 68 of" the vacuum tube 46...v is'connectcd to' ground through a portion 70A of the' potentiometer" 70 and the grounded adjustable tapv 72' thereon.. Bypass condensers 767 and 78' are' respectively' connected in shunt with sections 7 0a and 7 0b` ofgthe-pot'e'ntiometer. 70. The function of these bypass condensers wilLbetmorey clearlyl evidentV as the specification proceeds". The;lowerextremity'ofvthe potentiometerI 70 is connected witlrasourceofspositive potential St'l'through a dropping resistor 82;. and is further connected to the' cathode 8'4* oftheA'.. G; C. diode 86. Filter condenser'tlS is appropriately; connectedfrom the'cathode 84- of the diodel to` ground to reducev any voltage` fluctuations thatv tend; toi occur at that point. The anode 90 af theA. G. C. diode 86 is connected with the' A. G. C. diode' load 92anditsV associated integrating network comprising resistor 9'4 and' capacitor 9'6. These two elements of the integrating network are usuallyV selected to have' a time. constant whichV allows substantially' little discharge: of.v the condenser 965 during one vertical field period. The. voltage' 75 AIt may be. further. noticed thatthecombination action- V appearing across the storage condenser 96 may be conventionally connected with various radiofrequency and intermediate frequency amplifier grid circuits as shown through A. G. C. bus 98 to supplement the bias applied to the receiver amplifier sections 12 and 16.

The A. G. C. system associated with diode 86 shown in this. embodiment is of the double time constant. type well known to the art, with the input time constant composed of C and R972 the value of this time constant being relatively short', usually' in the order of 1 or 2 horizontal lines. Accordingly, the intermediate frequency signal applied to the second detector 28 is also applied through circuit path S3 andl coupling condenser 85 toL the anode 90 of the A. G. C. detector 86. As before described, the A. G: C. cathode 84 is connected to the low end of potentiometer 70 through section 7013i to ground as provided. by adjustable tap 72 thereon. Since a small value of bleeder current exists through the scction 70B of the potentiometer due to its connection through resistor 82 to rpower supply voltage 80, it is apparent that the cathode 84 will be' establishedr at somev positiveV potential above ground of a' value which will be very'nearly proportional to the value of the resistance 70B which is in turnA determined by the position on the potentiometer of tap'72`. Since the cathode 84 is aboveI ground potential, and the diode is connected' to` conduct on positive peaks of signal 30, the tap 72 on the potenti-V ometer 70; operates as a' form of a variable bias control for the A. G. C. diode and hence provides a variable threshold control for the A. G. C. system. When' the signal supplied by the I; F. amplifier- 1-8 is sulcientinamplitude to cause conduction of thev diode 86 there' will, of course, be developed a voltage drop across re-l sistor 92 due to diode current. Examination will show thatv this voltage drop is in theY proper direction to cause the A. G. C. bus 98 to assume increasingly negative potenti'alval'ue with respect to ground as the signal intensity applied to theY diode 86 increases. In accordancel with well known A. G; C. system action, this increasev in negative voltage on A. G. C. bus 98 resulting from an increase in signal amplitude, tends to reduce the gain of the' various amplifier sections connectedV tothe A. G. C. bus, thereby' tending to minimize the voltage iiuctua-v tions at'the output ofthe I'. F. amplifier which occur due tovariations' inA the signal strength applied to the R. F. ampli'erIZ'.

Potentiometerl 7l) not only acts as a variableV tl'lresholdy control forthe A. G'; C. diode 36 but since its upperext'remity'7tlA is connected withl the cathode 68` of the video"l amplifier 46, the grounded` adjustable tapv 72 also acts to" control the amount of resistance' in the cathode circuit commento the grid circuit, which in turn establishes the degreel of degeneration applied tothe video`V amplifier tube 4'6i Thus'. as the tap 72 is moved upwardly on thesec'- tion 70A of the potentiometer 70, the resistance in the cathode circuit tendsto' decrease, thereby' de'cre'as'ingqthe' percentageot4 degeneration and increasing the gain of the video amplifier stage. This increase of gain, however, also' is attendedby an' increase of' the value of resistance 70B followingthe' change of tap 72 and hence increases. the" value of thresholdvoltageapplied to the A. G. C. diode 86.

Therefore', under the condition of restricted'v A`. G. C. action. such, for example, as would be provided by dioder 86, a suflicient increase in signal intensity applied to the R. F. amplifier 12' will'visibly cause an increase in picture contrast as viewed on kinescope 58, although such azchange will be'limited-in extentby' the A. G. C. action.. It will,

therefore, be necessary to decrease the gain ofthe video amplifier stage.46 andin so doing tap 72 will be movedV downwardly so as to increase the degeneration. in` the cathode circuit; andas described, will automatically decrease the amount of A. G. C. thresholdcontrol voltage'v ofthe degenerative volume control and threshold control tends to expand the range of the control actionsupplied by the potentiometer 70 over that realizable if the potentiometer were connected solely as a degenerative control. This action is understood by remembering that in the previous examples cited, the manipulation of tap 72 so to reduce the gain of video amplifier 46 also decreases the threshold voltage applied to the diode 86. Thus for a given peak to peak amplitude of vsignal applied to the R. F. amplifier 12 reduction of the gain of the video ampliiier 46 will produce an increase in negative A. G. C. control voltage which further acts to reduce the overall gain of the television receiver. The reverse of this action obtains when tap 72 is adjusted to increase the gain of the receiver.

. 'Ihe potentiometer 70 also serves in another capacity in cooperation with bypass condensers 76 and 78, namely that of a degenerative form of variable high frequency boost arrangement.

Figure 2 shows a number of frequency response curves for the video amplifier 46 under operating conditions corresponding to various positions of tap 72 on the resistor 70. Figure 2a shows the frequency response to be quite at for the adjustment of potentiometer 70 to yield maximum gain for the video amplifier stage. Maximum gain, of course, is to be realized when the tap 72 is positioned at the upper extremity of the potentiometer so as to directly connect the cathode 68 to ground potential, thereby imposing a minimum of degeneration, and such a position may be said to correspond eifectively to zero resistance position of the potentiometer 70. Operation under these conditions would correspond to necessary receiver operation when under the influence of extremely weak signals, and as such, provides no high frequency boost due to the probability of an overall decrease in signal noise ratio. As the tap 72 of potentiometer 70 is moved' downwardly so as to increase the degeneration and decrease the gain of the receiver, the frequency response characteristics of the video amplilier approach that shown in Figure 2b. Here there is evidenced a slight drop in low frequency response with an effective rise in high frequency response due to the lower impedance of capacitor 76 at higher frequencies which provides less degeneration at the higher frequencies. The response curve shown in Figure 2b is approximately that obtained when tap 72 is positioned to include about one-quarter of resistor 70 in a cathode circuit of the vacuum tube 46. Figure'Zc' shows the frequency response characteristic for the video amplifier when tap 72 is positioned to the fixed tap ofA potentiometer 70 and very closely represents the maximum high frequency boost obtainable with this arrangement since the time constant of condenser 76 and the upper fixed section of potentiometer 70 is preferably made the same as the time constant of the series value of the condensers 76 and 78 in combination with the total resistance of potentiometer 70. Since this is true, positioning of tap 72 at any point on the potentiometer more distant from the cathode 68 than the fixed tap 74, merely decreases the gain of the video stage with little noticeable change in frequency response characteristics. This effect is borne out by experimentally derived frequency response curves 2d and 2e.

The second detector arrangement described in connection with Figure 1 employs a D. C. connection between the second detector 28 and the video amplier control grid 44. Under these particular conditions a further advantage realized from the present invention, as shown in connection therewith, in that of suitable compensation for changes in grid 44 operating bias as the signal strength, and consequently the diode current of diode 28 changes. It is apparent from the connections shown that as the signal strength increases with the subsequent increase in the diode current through resistor 40, the grid 44 will t tend to become biased more positively and hence change the operatingY bias on the-grid 44 in the direction of plate current saturation, which if allowed to proceed lunchecked would impose serious distortion on the television synchro-- nizing pulses. However the potentiometer 70 is further arranged so that the increase in signal will be not only compensated by a manual controlled degenerative reduction in gain of the vacuum tube 46 but also as tap 72 is moved in a direction away from the cathode 68 an increase in negative operating bias is provided so as to maintain the grid 44 operating point in a more desirable position of the tube plate current characteristics.

lf a D. C. connection from the second detector to the first video amplifier is not desired, the present invention may still be practiced as shown in Figure 3 where like elements are assigned like numerical designation. The only diiference in the operation of the arrangement from that shown in Figure l lies in the supply of a fixed bias potential to the video amplifier control grid 44 through-a resistor 51 connected between said grid and a negative bias -supply terminal 50. The operation of the present invention to provide a combination control of video amplifier gain, video amplifier, frequency response, andA A. G. C. threshold voltage remains unchanged. However, as is apparent, adjustment of potentiometer 70 no longer serves to more closely maintain a predetermined grid operating point in tube 46 due to the A. C. coupling of the detector 28 to the grid 44 through coupling condenser 33. i

In an experimental setup in which the highest video frequency of interest was 3 mc. the following values were assigned the parameters shown in Figure l:

l,It was from this experimental arrangement that the data constituting the basis for the curves of Figure 2 were obtained. In the curves the designation fn rep-- resents the 3 mc. maximum which the system was designed to handle.

From the foregoing description of the method, practice and operation of the present invention, it is seen' that the applicant has provided a novel form of single combination control for superheterodyne radio receivers employing restricted A. G. C. action said control olering simultaneous manual adjustment of the frequency response characteristics of the receiver, the automatic gain control threshold bias, and the overall gain of the receiver; and varies these characteristics in such a way as to provide improved receiver performance.

What s claimed is:

l. In a television receiver, the combination of; a

source of demodulated video signals having both high l and low frequency components; an amplifier device having at least an input electrode, an output electrode and a common electrode against which said input and output electrodes may be electrically referenced in the operation of said amplifier device and through` Which .Operation current for said amplifier device ows, said device being characterized in that the gain and op- Hating current thereof may be controlled by varying the direct current potential caused to appear between said input electrode and said common electrode; a source of power supply potential having a first and second terminal across which appears an operating potential for the actuation of said amplifier; first resistance means operatively connected from said input electrode to said first power supply terminal to form an input circuit for said amplifier; a direct current conducting output circuit operatively connected between said output electrode and said second power supply terminal to develop amplified versions of signals caused to drive said input circuit; means operatively coupling said source of video signals to Asaid input circuit for driving said amplifier device input circuit; a second resistance means having two extremities and a movable tap intermediate said extremities; a capacitance means connected across the extremities of said second resistance means to form a time constant combination having one access terminal defined by one extremity of said second resistance means and a second access terminal defined by said movable tap; a direct current connectionv between one access terminal of said time constant combination and said amplifier common electrode; and a direct current connection between the other access terminal of said time constant combination and said first power supply terminal such that average current flowthrough said common electrode produces a direct current voltage drop across said access terminals whereby adjustment of said tap provides a range of potential change between said input electrode and common electrode which operatively varies the gain of said amplifier between a maximum value and a minimum value, the value of said capacitance means being` so related to the value of said second resistance means that the amplitude of video signal high frequency comf ponents appearing in said output circuit is accentuated under conditions of minimum gain over their relative amplitude under conditions of maximum gain. Y

2. In a television receiver, the combination of a source of demodulated video signals having both high and low frequency components; an amplifier device having at least an input electrode, an output electrode and a common electrode against which said input and'output electrodes may be electrically referenced in the operation of said amplifier device and through which operating current for said amplifier device flows, said device be ing characterized in that the gain and operating current thereof may be controlled by varying the direct current potential caused to appear between said input electrode and said common electrode; a source of power supply potential having a first and second terminal across which appears an operating potential for the actuation of said amplifier; first resistance means operatively connected from said input electrode to said first power supply terminal to form an input circuit for said amplifier; a direct current conducting output circuit operatively connectcd between said output electrode and said second pow er supply terminal to develop amplified versions of signals caused to drive said input circuit; means operatively coupling said source of video signals to said input circuit for driving said amplifier device input circuit; a second resistance means having at least a first and second scctions in series relation to one another with an adjustable tap operatively connected with one of said sections; a capacitance means connected in shunt with at least a portion of said second resistance means to form a time constant circuit a first access terminal of which is defined by one extremity of said resistance means and a second access terminal by said adjustable tap; a means direct current connecting said time constant access terminals in series between said amplifier device common electrode and saidifirst power supply terminal such that current flow through said common electrode produces a P voltage drop across said access terminals whereby adjustment of said tap produces a time constantchange in the circuit from said common electrode to said power supplyv as well as a potential change between said input electrode and common electrode whereby the gain of said amplifier device may be varied over a useful range by means of positioning said adjustable tap, the value of said capacitance means and said resistance means being so related that the response of said amplifier circuit to the higher frequency components is changed as in response Vto adjustment of said tap and is minimized under those conditions where said tap is adjusted to provide maximum gain in said amplifierdevice.

3. In a television receiver, the combination of: a source of demodulated video signals having both high and low frequency components; an amplifier device having at least an input electrode, an output electrodeand a common electrode against which said linput and output electrodes may be electrically referenced in the operation of said amplifier device and through which operating current for said amplifier device Hows, saidv device being characterized in that the gain and operating current thereof may be controlled by varying the direct current potential caused to appear between said input electrode and said common electrode; a source of power supply potential having a first and second terminal across which appears an operating potential for the actuation of said amplifier; first resistance means operatively connected from said input electrode to said rst power supply terminal to form an input circuit for said amplifier; a direct current conducting output circuit operatively connected between said output electrode and said second power supply terminal to develop amplified versions of signals caused to drive said input circuit; means operatively cou' Y pling'said source of video signals to said input circuit for driving said amplifier device input circuit; a second resistance means; a capacitance means connected in shunt with said second resistance means to form a time constant circuit; direct current connections placing at least a portion of said second resistance means between said common electrode and said first power supply terminal so that average current flow through said-common electrode produces a direct current voltage drop'across said portion of said second resistance means such that said voltage drop appears as a direct current potential between said input electrode and common electrode; means for varying the value of that portion of said second resistance means connected between said common electrode and said first power supply terminal whereby the value of direct current potential appearing between said input electrodes and common electrode may be varied to produce a change in the gain of said amplifier over a range having a maximum anda minimum, the value of said capacitance means being so related to the value of said second resistance means thattthe amplitude of video signal high frequency components appearing in said output circuit is accentuated under conditions of minimum gain over the relative amplitude of high frequency components under conditions of maximum gain.

4. In a television receiver, the combination of: a source of demodulated video signals having low and high frequency components; a video amplifier comprising an amplifier device having electrodes corresponding to an anode, cathode and control electrode; a power supply for said video amplifier having positive and negative terminals; resistance means connected between said control electrode and said negative terminal to form an input circuit to said video amplifier; means operatively coupling said signal source to said input circuit; output circuit means operatively connected between said anode and positive terminal for developing video output signals input circuit; a resistance means having at least one movable tap thereon and one fixed tap; a direct current connection from one extremity of said resistance to said cathode; a direct current connection from said tap to said negative terminal; and capacitance means connected in shunt with portions of said resistance means such that resistance defined by said tap forms at least in part a time constant circuit degeneratively situated in said ampliier such that the degree of degeneration and the time constant values of said time constant circuit may be varied by moving said tap, said capacitance means being further so valued in relation to said resistance means that the degeneration provided by said time constant circuit is frequency selective to accentuate the high frequency response of said video amplier at positions of said tap providing maximum degeneration.

References Cited in the ile of this patent UNITED STATES PATENTS Beers Sept. 28, Rechnitzer Dec. 7, Ferris Apr. 18, Brown Sept. 21, Roberts Feb. 17, Maxwell Feb. 6, Crooker et a1. Feb. 5,

FOREIGN PATENTS France Sept. 13, 

